Section 100(b) in The Building And Other Construction Workers '(Regulation Of Employment And Conditions Of Service) Central Rules, 1998
(b)when dry ingredients of concrete are being mixed in confined spaces such as silos,-(i)the dust shall be exhausted at the time of such mixing; and(ii)in case the dust cannot be exhausted, as specified in sub-clause (i), the building workers shall wear respirators at the time of such mixing;